1. Field of the Invention
Preferred embodiments of the present invention generally relate to a sheet processing apparatus and a sheet conveyance method, and more particularly, to a sheet output technique for jogging and stapling a stack of sheets and outputting the stack of sheets while keeping the jogged state.
2. Discussion of the Related Art
In a sheet processing apparatus, a sheet after image formation is ejected from an image forming apparatus and temporarily stacked on a sheet stacking tray. Then, a jogging unit jogs a stack of sheets on the sheet stacking tray, and if instructed, a stapler staples the jogged stack of sheets. Subsequently, the stack of sheets is output by a sheet output nail to a sheet output tray from the sheet stacking tray.
For example, FIG. 42 shows a conventional sheet processing apparatus including a sheet jogging unit and a sheet output unit. FIG. 43 is a side view showing a magnified F area shown in FIG. 42. The sheet jogging unit includes an end fence 101 for jogging a stack of sheets in a sheet conveyance direction and a jogger fence 105 for jogging the stack of sheets in a direction perpendicular to the sheet conveyance direction. The sheet output unit includes a sheet output belt 107 extended over an output roller 103 and a driven roller 108 in parallel with the sheet conveyance direction and in a center zone of a processing tray 102, and a pair of sheet output nails 106 provided and projected on the sheet output belt 107.
A sheet ejected onto the processing tray 102 through a conveyance path, which is not shown, moves back by its own weight and a reverse roller, which is not shown, toward the end fence 101. A back end of a stack of sheets is jogged by touching the end fence 101. Each time when one sheet is ejected, the jogger fence 105 touches both sides of the sheet and jogs the stack of sheets in a width direction of the sheet such that a center of a sheet conveyance path and a center of the sheet coincide. When the stack of sheets to be output in one operation is jogged on the processing tray 102, the output roller 103 drives the sheet output belt 107, and the sheet output nail 106 pushes up the back end of the stack of sheets and conveys the stack of sheets to a downstream side of the sheet conveyance direction farther than the output roller 103. When an image forming apparatus instructs stapling the stack of sheets, a stapling unit 104 staples the back end of the stack of sheets before the sheet output nail 106 conveys the stack of sheets. When the image forming apparatus does not instruct stapling the stack of sheets, the stack of sheets is conveyed without being stapled to the downstream side. When being conveyed without being stapled, the stack of sheets may not maintain the jogged state while being conveyed.
For example, several known techniques have been proposed for outputting a stack of sheets while maintaining a jogged state thereof. In a technique shown in FIG. 42, a hook bent toward a sheet output direction is provided at a front end of each of the sheet output nails 106 for outputting a stack of sheets jogged and stacked on the processing tray 102, and a straightening member for pressing the front end of the stack of sheets is provided in an opposite side of the hook on each of the sheet output nails 106. In the technique, in order to maintain the jogged state of the stack of sheets, the front end of the stack of sheets is pressed toward a bottom of one sheet output nail 106 by using the straightening member of the other sheet output nail 106.
In another technique, in order to avoid curling of a stapled stack of sheets and keep a jogged state of a stack of sheet without stapling when the stack of sheets is output on a sheet output tray, an angle of the sheet output tray can be changed depending on whether or not the stack of sheets is stapled.
Each of the above two techniques employs a sheet stacking tray that inclines almost vertically, and a sheet output nail is required to convey a stack of sheets against gravity thereof. Another technique employs a horizontal sheet stacking tray as shown in FIG. 44. In the technique, a sheet output nail 8 conveys a stack of sheets that is stacked and jogged on an intermediate tray A in a horizontal direction and outputs the stack of sheets onto a sheet output tray B.
However, such techniques have drawbacks in that a stack of sheets is output on the sheet output tray B in a ragged state. When a sheet output face 8a of the sheet output nail 8 is vertical to a sheet stacking face A1 of the intermediate tray A, a back end of the stack of sheets that is output on the sheet output tray B is jogged along the vertical sheet output face 8a. The back end of the stack of sheets falls in a direction indicated by an arrow G and moves on the sheet output tray B toward a backboard 36 as shown in FIG. 45. At the moment, several lower sheets do not move due to gravity of upper sheets, and upper sheets only touch the backboard 36. As a result, the stack of sheets may not be output in the jogged state.
On the other hand, in the technique shown in FIG. 42 and FIG. 43, as the driven roller 108 is provided far below the processing tray 102, the stapling unit 104 requires to move so as to avoid the driven roller 108. When the stapling unit 104 moves along a width direction of a stack of sheets, a space for the stapling unit 104 to avoid the driven roller 108 is required. As a result, the conventional sheet processing apparatus shown in FIG. 42 increases in size. If the end fence 101 carries the stack of sheets to a position in which the stapling unit 104 does not interfere with the driven roller 108, the space for the stapling unit 104 to avoid the driven roller 108 is not required, and the size of the sheet processing apparatus may be smaller. However, in order to move the end fence 101, another drive unit for driving the end fence 101 is required. Thus, the sheet processing apparatus further increases in size and cost.